Magnetic resonance imaging (MRI) is a technique that utilizes a nuclear magnetic resonance phenomenon (NMR) for finding out the local distributions of the nuclear density and nucleus-related NMR properites of an object or the physical and chemical properties having an effect thereon. Such NMR properties include e.g.: longitudinal relaxation (characterized by longitudinal relaxation time T1), transverse relaxation (characterized by transverse relaxation time T2), relaxation in spinning coordinates (characterized by relaxation time (T1rho), chemical shift, and coupling factors between nuclei. The physical phenomena having an effect on NMR properties include e.g.: flow rate, diffusion, paramagnetic materials, ferromagnetic materials, viscosity and temperature.
Magnetic resonance and magnetic resonance imaging methods and applications have been described in a number of references: Wehrli F. W., Shaw D., Kneeland B. J.: Biomedical Magnetic Resonance Imaging, VCH Publishers, Inc., New York 1988, Stark D. D. and Bradley W. G.: Magnetic resonance imaging, C. V. Mosby Comp., St. Louis 1988, Gadian D. G.: Nuclear magnetic resonance and its applications to living systems, Oxford Univ. Press, London 1982, Shaw D.: Fourier transform NMR spectroscopy, Elsevier, Amsterdam 1984, Battocletti J. H.: NMR proton imaging, CRC Crit. Rev. Biomed. Eng. vol. 11, pp. 313-356, 1984, Mansfield P. and Morris P. G.: NMR imaging in biomedicine, Adv. in magnetic resonance, Academic Press, New York 1982, Abragam A.: The principles of nuclear magnetism, Clarendon Press, Oxford 1961, Lasker S. E. and Milvy P. (eds.): Electron spin resonance and nuclear magnetic resonance in biology and medicine and magnetic resonance in biological systems, Annals of New York Academy of Sciences vol. 222, New York Academy of Sciences, New York 1973, Sepponen R. E.: Discrimination and characterization of biological tissues with magnetic resonance imaging: A study on methods for T1, T2, T1rho and chemical shift imaging, Acta polytechnica scandinavica EL-56, Helsinki 1986, Fukushima E. and Roeder S. B.: Experimental pulse NMR, Addison Wesley, London 1981, Thomas S. R. and Dixon R. L. (eds.) NMR in medicine: The instrumentation and clinical applications, Medical Physics Monograph No. 14, American Institute of Physics, New York 1986, Anderson W. A. et al: U.S. Pat. No. 3,475,680, Ernst R. R. : U.S. Pat. No. 3,501,691, Tomlinson B. L. et al: U.S. Pat. No. 4,034,191, Ernst R. R.: U.S. Pat. No. 3,873,909, Ernst R. R.: U.S. Pat. No. 4,070,611, Bertrand R. D. et al: U.S. Pat. No. 4,345,207, Young I. R.: U.S. Pat. No. 4,563,647, Hofer D. C. et al: U.S. Pat. No. 4,110,681, Savolainen M. K.: Magnetic resonance imaging at 0.02 T: Design and evaluation of radio frequency coils with wave winding, Acta Polytechnica Scandinavica Ph 158, Helsinki 1988, Sepponen R. E.: U.S. Pat. No. 4,743,850, Sepponen R. E.: U.S. Pat. No. 4,654,595, Savolainen M. K.: U.S. Pat. No. 4,712,068, Sepponen R. E.: U.S. Pat. No. 4,587,493, Savolainen M. K.: U.S. Pat. No. 4,644,281 and Kupiainen J.: U.S. Pat. No. 4,668,904.
Most often, the medical imaging utilizes the magnetic resonance of a hydrogen nucleus since the hydrogen nucleus has a high magnetic moment and since its content in a biological tissue is high. Hereinafter we shall follow the practice adopted in literature, wherein the hydrogen nucleus is referred to as a proton and the nuclei being examined are generally referred to as spin nuclei.
According to the prior art and referring to FIG. 1, an object P to be examined is placed in a magnetic field B.sub.o as homogeneous as possible (so-called polarizing magnetic field), the apparatus further including a signal coil C for detecting an NMR signal, said coil being connected to an NMR spectrometer L, the apparatus being provided, for coding positional information, with a gradient coil arrangement G, the current required thereby being supplied by gradient current sources GC controlled by the spectrometer.
It is prior known to subject an object being examined to a radio frequency radiation at a frequency different from that of nuclear magnetic resonance (off-resonance radiation) for thus saturating the magnetization of abundantly proteinaceous components. In the case of a biological tissue, the signal visible in magnetic resonance imaging most often originates from the protons of water or fat molecules. Typically, the transverse relaxation time T2 of this NMR signal is more than 30 ms. The relaxation time of a signal corresponding to the protons contained in proteins is less than 0,5 ms--too short for the proteins to be directly visible in typical magnetic resonance imaging. By applying the radiation different from the resonance frequency of the protons included in the water and fat molecules of a tissue it is possible to saturate the nuclear magnetization of the protons of said proteins without having a direct effect on the nuclear magnetization of the protons of fat and water.
Between the protons contained in proteins and the protons contained in water molecules there is a continuous interaction. Thus, saturation of the protons included in proteins has an effect on the nuclear magnetization of water molecules through a so-called magnetization transfer (MT) penomenon. This phenomenon can be utilized for studying interactions between the proteins, fat and water of tissues and to achieve in magnetic resonance imaging an improved contrast between different tissues. The magnetization transfer phenomenon has been described e.g. in reference S. D. Wolff and R. S. Balaban: Magnetic Resonance in Medicine, 10, 135-144 (1989).
The above-mentioned saturation phenomenon should not be mistaken for a so-called radiofrequency bleed). This phenomenon refers to the direct effect of off-resonance radiation on an object whose longitudinal relaxation time is T1 and transverse relaxation time T2: EQU Mf=MO*(1+T2.sup.2 *w.sup.2)/(1+T2.sup.2 *w.sup.2 +(96 B1).sup.2 *T1*T2), (1)
wherein Mf is magnetization after radiation, MO is the value of equilibrium magnetization, w/(2.pi.) is a difference frequency between radiation frequency and proper resonance frequency, B1 is the amplitude of a radiated alternating magnetic field and .tau. is the so-called gyromagnetic ratio of a nucleus being examined. A presumption in formula (1) is that radiation time is of the same order as longitudinal relaxation time T1. Generally, in magnetization transfer test conditions the difference frequency is selected in a manner that the direct effect of radiation, as indicated in formula 1, is minor if compared to the indirect effect occurring through proteins.
Inversion recovery (IR) is one of the techniques applied in magnetic resonance imaging. As shown in FIG. 2, the technique or method comprises an inversion pulse (IP) or some other similar action for producing inversion (e.g. adibatic rapid by-pass, composite pulses), the magnetization vector being turned 180.degree., a recovery time TI, the magnetization recovering towards its balanced value, as well as the actual imaging, the contrast of an image obtained depending on the degree the magnetization has had time to recover within the period of TI.
The recovery of magnetization can be described with a time constant T1 (longitudinal relaxation time): EQU M=M.sub.o (1-2 exp(-TI/T1)+exp(-TR/T1)), (2)
wherein M is the magnitude of magnetization after time TI has lapsed from inversion pulse, M.sub.o is a magnetization corresponding to equilibrium, and TR is the interval for repeated measurements. Relaxation time T1 depends on the chemical and physical properties of an object (e.g. various types of tissue) being imaged. In certain cases, the recovery of magnetization cannot be very well demonstrated by means of a single relaxation time, but even then the main features of the phenomenon can be described by means of a more simple formula 2. What is particularly essential in inversion recovery sequence is that with a certain selection of inversion time the magnetization of a sample possessing a certain T1 and hence also the obtained NMR signal is zero.
In medical imaging the above principle has ben exploited: the inversion time is selected in a manner that a certain type of tissue produces a zero signal, the tissue contrast between this and other tissues being significant. In normal imaging, however, it is not important whether some signal is exactly zero but the only significant point is the absolute intensity difference in relation to noise.
One interesting group of magnetic resonance imaging methods is associated with magnetic resonance angiography (MR angiography). An object in these methods is to maximize the contrast of blood vessels in relation to other tissues. If the contrast is sufficient, it is possible to apply a so-called projection method with the entire object being excited at the same time. Thus, the entire blood vessel system of an object is visible as an image similar to X-ray angiography.
The above-described MR angiography and the like projection techniques require that a signal emitted from other types of tissue than that of interest be nearly zero. In most cases, such a contrast can only be achieved by a combination of two or more images. For example, MR angiography can be carried out by first taking an image with moving spin nuclei issuing just a small signal and then an image with flowing spin nuclei putting out a normal signal. This is possible by means of a so-called GMN method or similar methods (see e.g. the reference C. E. Spritzer, R. A. Blinder: Magnetic Resonance Quarterly, 5, 205-227 (1989)). In the differential image of these two images the stationary spin nuclei disappear and only moving spins remain (e.g. blood flowing in blood vessels). Another possibility is to take an image before and after the injection of a contrast medium. The properties of blood change through the action of a contrast medium while other tissues generally remain unchanged. Thus, all that is visible in the differential image is the blood vessel system.
Two separate images involve certain problems. The movement of an object (e.g. a patient) can ruin the end result. The dynamics required of the entire system are enormous since, generally, a signal emitted by a tissue being examined is just a fraction of the entire signal.
Thus, it would often be desirable to create a situation in which the original signal would only originate from certain or certain desired types of tissue. The inversion recovery sequence is one way of reducing a signal originating from certain selected types of tissue even to zero. A problem is, however, that biological objects are most often complicated structures having a great variety of different relaxation times. It is not generally possible to select the inversion time TI in a manner that there is a zero signal coming from all other types of tissue except one particular type.
In addition to MR angiography, the above aspect applies in MR myelography which strives to clarify the distribution of cerebrospinal fluid in an object.
One application of magnetic resonance imaging is so-called perfusion sensitive imaging for visualizing microcirculatory phenomena. The widely applied method is based on the use of large so-called perfusion gradients. These gradients are on after the excitation pulse prior to signal collection and their amplitude and duration are selected in a manner that their effect on stationary spin nuclei is zero but on moving spin nuclei other than zero. The end result image is used to examine the variations in the amplitude or phase of a signal.